This invention relates to wire bond interconnection in semiconductor packaging.
Fine wires are commonly used to make electrical interconnections between contact sites in semiconductor packages. The contact sites may be, for example, an interconnect pad on a die and a bond finger on a lead frame or a bond site on a substrate. Typically a ball is formed at one end of the wire and the ball is bonded to a first contact site. Then the wire is carried to a second contact site, forming a loop, and is stitch bonded (wedge bonded) at the second site to complete the interconnection. The wire is then broken at the end of the wedge bond, forming a new free end on which a ball can be formed to repeat the process to form interconnections between another pair of contact sites.
Wire bonding is an automated process, employing a wire bonding tool that operates robotically according to programmed instructions. In a conventional wire bonding tool a bond wire, which is typically gold, is run off a spool and threaded through the lumen of a hollow, pointed ceramic capillary. A free end of the wire projects beyond the tip of the capillary. A molten ball is formed at the free end of the wire (a so-called free air ball), typically by employing an electrode near the wire to strike a high voltage electric arc. The bonding site and the capillary are heated, and the capillary is maneuvered to press the free air ball against the bonding site, and ultrasonic energy is applied through the capillary and the ball as the ball hardens and the bond is formed. Typically the ultrasonic energy is supplied to the capillary from an ultrasound source by way of a transducer.
The quality of the interconnection depends upon suitable combinations of materials constituting the wire and contact site, and upon suitable combinations of temperature, pressure, ultrasonic energy, and the timing of events during bond formation. In particular, it is important to apply an effective amount of ultrasonic energy in a controlled manner.